Apparatus for stretching a non-woven web or an orientable polymeric material

ABSTRACT

There is described in a preferred embodiment of the present invention, a process and apparatus for bi-axially stretching a non-woven web of synthetic fibers in a first and second station wherein the first and second stations are provided with sets of rolls having grooves parallel and perpendicular, respectively, to the axis of each set of rolls. The groove pattern of the rolls is generally a sinosoidal wave with distances between grooves of less than 1.0 millimeters times the web basis weight in grams per square meter.

This is a division of application Ser. No. 900,720, filed Apr. 27, 1978,and now U.S. Pat. No. 4,223,059 granted Sept. 16, 1980, whichapplication is a continuation of application Ser. No. 563,623 filed Mar.31, 1975, abandoned.

This invention relates to a novel process and apparatus for thestretching of non-woven and spun-bonded webs of synthetic fibers toimprove the strength thereof and more particularly to a novel processand apparatus for the bi-axial stretching of non-woven webs oforientable, polymeric fibers.

BACKGROUND OF THE INVENTION

Many non-woven and (continuous filament) spun-bonded fabrics or webshave found use in the market as substitutes for textile materials. Suchwebs consist of randomly laid fibers, either in the form of short staplelengths or continuous filaments, which are undrawn or only partiallydrawn, and have therefore not obtained their optimum strength.Conventional methods of web stretching, such as Godet-roll stretchingand lateral stretching in tenter-frames, do not lend themselves tostretching of such webs. In longitudinal roll stretching a narrowing ofthe web occurs, the individual fibers between bonding points do notundergo molecular orientation. In lateral tenter-frame stretching,tearing at the clamps frequently destroys the web. In the case ofnon-woven webs made from short length staple fibers, individual fibersbecome separated because the stretching tension exceeds the bondingstrength of the bonded cross-over points of the fibers.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide a novel process andapparatus for stretching a non-woven web of synthetic material.

Another object of the present invention is to provide a novel processand apparatus for bi-axially stretching a non-woven web of syntheticmaterial to substantially improve optimum strength.

Various other objects and advantages of the present invention willbecome apparent from the following detailed description of an exemplaryembodiment thereof with the novel features thereof being particularlypointed out in the appended claims.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a processand apparatus for selective stretching a non-woven web of syntheticfibers in a station provided with a set of grooved rolls. The groovepattern of the rolls is generally a sinosoidal wave having a distancebetween grooves of less than 1.0 millimeters times the web basis weightin grams per square meter. Accordingly, the web of synthetic material isstretched in a manner to affect uniform stretching between the bondingpoints of each individual fiber thereby producing a web of largerdimension in the direction of stretch.

In accordance with a preferred embodiment of the present invention,there is provided a process and apparatus for bi-axially stretching sucha web of synthetic fiber in a first and second station wherein the firstand second stations are provided with sets of rolls having groovesparallel and perpendicular, respectively, to the axis of each set ofrolls. The groove pattern of each set of rolls is such that the distancebetween grooves is less than 1.0 millimeters times the web basis weightin grams per square meter. The non-woven web of synthetic material isstretched in a manner to affect uniform stretching between bondingpoints of each individual fiber thereby producing a web of bi-axiallylarger dimension and correspondingly reduced base weight.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood by reference to thefollowing detailed description of an exemplary embodiment thereof inconjunction with the accompanying drawings wherein,

FIG. 1 is a schematic side elevational view of the first station of theapparatus and process of the present invention;

FIG. 2 is an isometric view of one of the rollers of the first station;

FIG. 3 graphically illustrates a sinosoidal curve;

FIG. 4 is a schematic side elevational view of the second station; and

FIG. 5 is a top view of the second station.

DETAILED DESCRIPTION OF THE INVENTION

Drive and support assemblies, timing and safety circuits and the likeknown and used by those skilled in the art have been omitted in theinterest of clarity.

Referring to FIG. 1 illustrating the first station of the process andapparatus of the present invention, there is provided a supply roll 10on which is mounted a non-woven web 12 of orientable polymeric fibers(continuous or discontinuous). The web 12 is coursed between a nip 14 ofa pair of rollers 16 having a plurality of grooves 18 parallel to theaxis of the rollers 16, as seen in FIG. 2. The web 12 is maintainedagainst the lower grooved roller 14 by a pair of press rollers 21 toensure that the velocity (ν₁) of the web 12 is substantially identicalto the surface velocity (ν_(l)) of the grooved rollers 16. The grooves18 of the rollers 16 are intermeshed like gears, as known to thoseskilled in the art. As the web 12 enters the nip 14, the web 12 assumesthe shape of a groove 18 and is stretched by a factor determined by thelength of the sinus wave "l" (See FIG. 3) of the groove divided by theoriginal length of the web "ω" between contact points of each respectivegroove tip, since the web is prevented from slipping by the pressrollers 21 to prevent the introduction of more material, as is morecommonly practiced in the corrugating art.

The draw ratio l/ω is calculated by the following equation where

a=πd/2ω, and the sinus wave of the groove is ##EQU1## Thus for d/ωratios of 1.0, 0.75 and 0.5 the draw ratios are 2.35, 2.0 and 1.6,respectively.

The web 21 after passage through the nip 14 of the rollers 16 is pulledaway by a pair of tension rollers 22 having a surface velocity (ν₂)greater than the surface velocity of the rollers 16, but not greaterthan a factor of the draw ratio affected in the nip 14 of the rollers16.

In accordance with the present invention, the length of the fabric istherefore increased by this factor. It is noted that the web does notundergo narrowing while being longitudinally stretched or extended, asis the case with conventional roller systems. It is apparent to oneskilled in the art that the web may sequentially pass through aplurality of pairs of grooved rollers 16 to further stretch lengthwisethe web 12.

If stretching is permitted to be carried out beyond the residualelongation of the individual fibers, the fibers will break and the webloose tensile strength. Strength per fabric basis weight is increasedconsiderably up to the point of breaking fibers. The maximum permissibledraw ratio can easily be determined by measuring the residual elongationof the individual fibers of the original web. For best results, thegrooves 18 of the rollers 16 should be as fine as possible, with groovedistance being increased, if heavy basis weight factors are to beoriented. From experience, good results are obtained, if the distancebetween grooves (in mm) is less than 1.0 times the fabric basis weight(in gram/m²). The larger the groove distance, the more fiber cross-overpoints are being separated or broken thereby weakening the fabricstructure. In the case of non-woven webs made from short staple fibers,groove distance (d) should be one-half of the fiber length or less. Ifthe groove distance (d) is larger, fibers are separated out by breakingrather than stretched between the grooves, and the web thereby becomesvery weak.

Referring now to FIG. 4, the longitudinally stretched web 20 from thefirst station including press rollers 21 is introduced into a nip 24formed by a pair of rollers 26 having a plurality of grooves 28 parallelto the circumference of the rollers 26 in a second station of theapparatus. The web 20 is caused to be coursed into the nip 24 by a pairof press rollers 30 which holds the web 20 against the lower roller 26to thereby prevent the web 20 from narrowing prior to introduction. Oncein the nip 24, the web 20 assumes the shape of the groove pattern andbecomes stretched by a factor of the draw ratio determined in a mannersimilar to the draw ratio discussed with reference to FIG. 1. In thesecond station, i.e., lateral stretching, the web 32 is wound up atabout the same velocity as the feed velocity. The crimp pattern isflattened out by stretching the web 32 laterally by means of tenterclamps or curved Mount Hope rolls, generally indicated as 34 such asknown and used by one skilled in the art with the product beingcollected on a roller 36. For best results, the longitudinal and lateralstretching steps are repeated alternately through multiple passes eachhaving a relatively low draw ratio, until the total permissible drawratio is reached. The number of longitudinal and lateral passes, as wellas the extent of the stepwise draw ratios, can be chosen so that a finalfabric is obtained with uniform properties balanced in both directions.

With the process and apparatus of the present invention, non-woven andspun-bended webs and fabrics can be produced having a much highertensile tear strength (expressed as grams per centimeter) and tensiletear length (expressed as meters) per basis weight than unstretchedfabrics. The fabric basis weight is decreased during the stretchingprocess by a factor of the area draw ratio (=longitudinal draw ratio xlateral draw ratio). The denier of the individual fibers is reducedsimilarly, resulting in a web of still bonded but finer fibers. Fabricsprocessed by the present invention are softer and drapier than theoriginal material and result in completely new types of products.

EXAMPLES OF THE INVENTION

Operation of the process and apparatus is described in the followingexamples which are intended to be merely illustrative and the inventionis not to be regarded as limited thereto.

EXAMPLE I

A randomly laid web of partially oriented polypropylene fibers(cross-over points of the filaments being most fused during melt blowingstep of web formation) produced by a process similar to that describedin U.S. Pat. No. 3,849,241 having the following properties is processedin accordance with the present invention:

    ______________________________________                                        Basis weight:       100 gm/m.sup.2                                            Filament denier:    1-3                                                       Tear strength (tensile)                                                                           785 gm/cm. (linear)                                       Tensile tear strength:                                                                            785 meters                                                ______________________________________                                    

The web is introduced into rollers 16 having a surface velocity of 3.05meters per minute. Each groove is formed with a depth (d) of 3 mm andwith a spacing (ω) of 4 mm thereby resulting in a draw ratio of 2.0. Theweb 20 is pulled from the nip 14 by tension rollers 22 having a surfacevelocity of 6.10 m per minute. The web is subsequently passed throughsimilar longitudinal and lateral stretch stations until a total drawratio of 16.0 is reached at which point the web exhibited the followingproperties:

    ______________________________________                                        Basis weight:       6.2 gm/m.sup.2                                            Filament denier:    0.1-1                                                     Tear strength (tensile):                                                                          98.4 gm/cm (linear)                                       Tensile tear length:                                                                              1587 meters                                               ______________________________________                                    

The tensile tear length per basis weight (expressed as meters) of theproduct is 1587 as compared with that of 785 of the startingpolypropylene fibrous material.

EXAMPLE II

A commercially available spun-bonded web of continuous polyester fiberhaving the following properties is processed in grooved rollers having adepth of 2.5 mm and a width of 5.0 mm (draw ratio is 1.6):

    ______________________________________                                        Basis weight:       63 gm/m.sup.2                                             Filament denier:    15                                                        Tear strength (tensile)                                                                           1102 gm/cm (linear)                                       Filament elongation:                                                                              250%                                                      ______________________________________                                    

The web is introduced into the rollers 16 at a surface velocity of 3.05meters per minute and is withdrawn by tension rollers 22 at a velocityof 4.88 meters per minute. After passage through two completestretchings operations (both including longitudinal and lateralstations), the web exhibited the following properties:

    ______________________________________                                        Basis weight:         25 gm/m.sup.2                                           Filament denier:      6                                                       Tear strength (tensile)                                                                             787.4 gm/cm                                             ______________________________________                                    

Tensile tear length (meter) of the product is 3149.6 meters as comparedto 1749.2 of the starting polyester material.

EXAMPLE III

A latex bonded randomly laid non-woven web consisting of 1.27 cmpolyester staple fibers having the following properties is processed inaccordance with the present invention:

    ______________________________________                                        Basis weight:       32 gm/m.sup.2                                             Filament denier:    7                                                         Tear strength (tensile):                                                                          157.5 gm/cm (linear)                                      ______________________________________                                    

The respective rollers were formed with grooves having a depth of 1 mmand a width of 3 mm (draw ratio 1.8). The web after bi-axiallystretching followed by pressing between calendar rolls exhibited thefollowing properties:

    ______________________________________                                        Basis weight:      18 gm/m.sup.2                                              Filament denier:   4.0                                                        Tear strength:     177.2 gm/cm (linear)                                       ______________________________________                                    

The tensile tear length (meter) of the product is 984.4 as compared with492.2 of the starting material.

While the present invention has been described with reference to thepassage of a web through a first longitudinal stretching station andthence a lateral stretching station, it is apparent that such stationsmay be altered with the web being first introduced into a lateralstretching station. Further, the apparatus and process of the presentinvention may be arranged to provide for longitudinal stretching to thelimit of the material capabilities prior to lateral stretching.Additionally, it is contemplated that in some instances that it isdesirable to stretch the material solely in either a longitudinal orlateral direction.

While the invention has been described in connection with an exemplaryembodiment thereof, it will be understood that many modifications willbe apparent to those of ordinary skill in the art and that thisapplication is intended to cover any adaptations or variations thereof.Therefore, it is manifestly intended that this invention be only limitedby the claims and the equivalents thereof.

What I claim is:
 1. An apparatus for bi-axially stretching a non-wovenweb of orientable polymeric material which comprises:(a) a first stationmeans for stretching said web in a first direction and including a firstset of interdigitating rollers formed with grooves having a distancetherebetween of less than one millimeter times the web basis weight ingrams per square meter, said first set of interdigitating rollersstretching incremental portions of said web in a first direction; (b)first regulator means for controllably introducing said web into saidfirst set of interdigitating rollers; (c) a first take-up means forelongating said web in said first direction upon withdrawal of said webfrom said first set of interdigitating rollers; (d) a second stationmeans for stretching said web in a second direction and including asecond set of interdigitating rollers formed with grooves having adistance therebetween of less than one millimeter times the web basisweight in grams per square meter, said second set of interdigitatingrollers stretching incremental portions of said web in a seconddirection; (e) a second regulator means for controllably introducingsaid web into said second set of interdigitating rollers; (f) a secondtake-up means for elongating said web in said second direction uponwithdrawal of said web from said second set of interdigitating rollers;and collecting means for receiving said web of bi-axially orientatedmaterial.
 2. The apparatus as defined in claim 1 wherein said first andsecond set of interdigitating rollers are formed with grooves which areparallel and perpendicular, respectively, to said first and second setof interdigitating rollers.
 3. The apparatus as defined in claim 2wherein said regulator means include a roller rotating at substantiallythe same rotational velocity as that of an associated interdigitatingroller.
 4. The apparatus as defined in claim 3 wherein said firsttake-up means are press rollers operated at a rotational velocityproportional to the draw ratio effected in said first set ofinterdigitating rollers.